Stabilization of cured polymers



Patented Feb. 25, 1947 UNITED STATES PATENT 2,416,282 STABILIZATION F CURED POLYMERS Burnard S. Biggs, Summit, N. J assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y.,- a corporation of New York No Drawing. Application April 30, 1943, V Serial No. 485,163 I This'invention relates to methods of improving the life and heat stability of the infusible, in-

11 Claims. (01. 260-45) soluble substances prepared by heating high mo- I lecular weight, thermoplastic polyesters with acyl peroxides.

The amount of benzoyl peroxide, or other acyl' peroxide, required for curing varies fromv about .5 per cent by weight or lessfor polyesters containing a substantial proportion of olefinic bonds and no inactivating substances to about 25-per cent for fully saturated polyesters heavily loaded with channel carbon black Those polymers cured with large amounts of peroxide,particularly those cured with more than about 5 per cent peroxide deteriorate much more rapidly in tensile strength and modulus than those cured with smaller'amounts, particularly when exposed to moisture and elevated temperatures.

The present invention is based upon the dis-.

covery that the stability of these curved polyesters can be increased by heating the cured polyesters in the absence of oxygen, preferably under a vacuum, ,for a period of time. In general, it is desirable to heat at a temperature aboveabout 125 C.,

under an absolute pressure of the order of several millimeters of mercury. A temperature of about 150 C. is ordinarily found very suitable, particularly when benzoyl peroxide is the curing-agent.

Pressures up to about or millimeters of mercury are ordinarily suitable although the lowest possible pressure is the most desirable; Stabilization may also be achieved by heating in an inert gas, such as nitrogen or hydrogen at atmospheric pressures, but a considerably longer period of heating is required. The time for which the heating is continued depends upon the heating temperature, the amount of peroxide used forcuring,

the nature of the peroxide and the degree of sta:

bilization desired. When the cured polymer is an sheet, heating for two or. three hours under a' vacuum will ordinarily give suitable results,

The stabilizing treatment referred'to abov is effective for any rubber-like or micro-crystalline substance obtained bycuring, with an acylperoxide, a high molecular weight polyester which is either normally a viscous liquid at'room temperatures or normally a micro-crystalline solid at. room temperatures which is capable'flof being reduced to a viscous-liquid upon heating.

These polyesters may be strictly linear poly esters of highmolecular weight containing no non-benzenoicl carbon-to-carbon' unsaturation which'are prepared by superesterification .of a' glycol with a dlcarboxylic acid or of a hydroxy acid with itself in a manner similar for instance to that described for crystalline polyesters in U. S. 1 Patents 2,071,250 and 2,249,950. Polyesters which I are essentiallysimilar but whichcontain limited amounts of olefinic unsaturation are also suitable.

These partially unsaturated polyesters may be' prepared in the same manner as the fully s'aturated polyesters except that one or more of the ingredients of the reaction' mixture from 'which they are prepared contain a proportion of unsaturated carbon-to-carbon bonds.

Theipresent invention is most concerned with .the stabilization of those substances of relatively article possessing a shape having a large surface area as compared to its volume, such as a'thin high tensilestrength produced by curing -polyesters which have achieved an extremelyhigh de gree of linear growth, although obviously-sud stancespossessing aconsiderably lowerdegree of Y polymerization will also benefit from this stabilization. With the strictly linear polyesters prf e-t pared from 'glycols and dicarboxylicz'acids" containing no non-benzenoid unsaturation or from monohydroxy monocarboxylic acids containing no non-benzenoid unsaturation, the degree oi linear growth is measured 'directlyby. the mo,-

lecular weight of-the polyester since theoretically .3 each molecule is made up of a single long chain. There is a relatively sharp increasein the'tens'ile- "strength of the cured polyesters when the ,mof lecular weights of the polyesters from whichthey are prepared achieve and exceed molecular weights in the vicinity of 8,000'to 10,000, as estimated by the Staudinger viscosity method lsini ear polyesters of such molecular weights'ordinar i ypossess intrinsic viscosities in chloroform'oiat least .4. Linearpolyesters will also possess such I molecular weights if they contain anav ragecof at least-.500 or 600 atoms in their linear chains or if they contain at least'98 ester groups-for each total ester, hydroxyl and carboxylgroups in thepolyester (98 per cent of theoretical complete esterification) v Polyesters formed from reactants, at least One of which contains olefinic unsaturation, will pos 'sess these high degrees of linear growth associated with high tensile strength if they contain at least 98 ester groups per 100 total ester, hydroxyl and carboxyl groups in the polyester. In order to produce the high degree of 'esterification'or condensation indicated above, the re-,

actants from which the polyesters are produced" must be subjected to a prolonged heating operation under conditions such as to remove the reaction by-products continuously and effectively,

as described for instance in U. S. Patents; 2,071,250 and 2,249,950. The reaction by-prod-, nets are most efiectively removed by bubbling an inert gas, such as dry, oxygen-free hydrogen, through the reaction mixture until esterification, or condensation has proceeded to the desireddev gree of completion, with or Without the application of reduced pressure.

'I'he process .of the present invention is applicable to cured polyesters derived from any glycols and, dicarboxylic acids or any monohydroxyv monocarboxylic acids'which are capable of esteri- 1 fying to form linear chains having lengths of the order desired, It is applicable to such polyesters in which the linear ester chains are made up of '7 divalent hydrocarbon radicals joined by ester Typical saturated glycols and dicarlinkages. boxylic acids are described-in U. 5. Patent 2,071,250. ,Those most commonly used are the polymethylene glycols and dicarboxylic acids and the alkyl substituted, polymethylene glycols and acids. Unsaturation may be introduced by subor mesaconic acids, for a portion of the saturated ,acids, or by substituting a non-conjugated unsaturated dicarboxylic acid, such as dihydromu conicacid, for all or apart of the saturated acid.

Similarly, a glycol containing olefinic unsaturathe saturated glycol or a hydroxy acid containall or part of a saturated hydroxy acid.

As examples of polymers which may be treated according to the present invention may be mentioned those produced by curing polyesters preparedby esterifying sebacic acid or succinic acid witha, mixture of an alkyl substituted polymethylenexglycol, such as isopropylene glycol, and a polymethylene glycol, such as ethylene glycol.

Similar polymers in which maleic acid replaces the succinic or sebacic acid to an extent of about 3 mol per cent, for instance-,: may also be stabilized in accordance with the present invention. These polymers arerubber-like at room temperjatures if the isopropylene glycol constitutes at least 80 mol per cent of the total glycol entering sebacate 'or seba cate-maleate polyesters or if the isopropylene glycol constitutes at least 50 mol per cent of the total glycol entering the succinate or succinate-maleate polyesters.

1 a In order to produce polyesters of the high degree of linear growth referred to above, it is necessary to limit the amount of conjugated unsaturation present in the reaction mixture so that the resulting theoretical polyester which would be produced, if no cross-linking occurred, would conitain lessthanabout five such olefinic bonds per 400 atomsinthe linear chain and preferably less than about two such bonds per 400 atoms in the linear chain. In general, when'the unsaturated stituting a conjugated unsaturated dicarboxylic, 3 acid, such-as muconic, maleic, furnaric, itaconic tion may be substituted for a portion or all of ing olefinic unsaturationmay. be substituted for polyesters ofv the high degree of linear growth referred to above areproduced by the esterifica tion of aiglycol with-a mixture of a saturated dicarboxylic acid, such as sebacic or succinic, and

' an unsaturated dicarboxylic acid, such a maleic,

;obviouslybeemployed when it is not desired. to

produce polyesters of such 1 a high degree ofcondensation, which are fluid or fusible.

As I stated; above, 1 polyesters produced as described are cured by introducingian acyl peroxide in anysuitable imannerand-heating to a temperature at which curing takes place at a practical rate. Benzoyl peroxide is the most effective curing agent yet found, but other acyl'peroxides, such as lauryl peroxide,. are suitable although less effective. If the polyester is sufficiently fluid or plastic to be milledon cold rolls or can be reduced to a fluid or plastic state by heating to a temperature sufiiciently low' that no curing takes place, the peroxide in a finely divided state is milled directly into the polyester. If the polyester can notbe reduced to afiuidstate below the curing temperature, the peroxidemay be dissolved ma solution of the polyester and the solvent may-be subsequently evaporated.

The amount of curing-agent required to give maximum tensile strengtlnwhen the cured prod-. uct is a'rubber-like substance,varies with the composition of polyester, being afiected both by the nature of the saturated components and by the amount of unsaturation. When a given polyester is cured with increasing amounts of 'benzoyl peroxide, the tensile strength rises Very sharply, passes through a peak and then decreases slowly. In saturated polyesters thislpeak is quite broad the optimum 'amountsbeing'from 3 .per cent to "6 per cent'for sebacate polyesters and from 5 percent to'8 per cent for succinate polyesters. As the amount of unsaturation increases, the peak becomes much sharper and theactual amount of peroxide required for curing becomes much less. Thus fora sebacate polyester in which 4 mol per cent of the sebacic acid is replaced by maleic acid, the optimum amount of peroxide" is from .5 per cent to'l' percent. Fora corresponding succinate maleate, the optimum amount is from 1 per cent to 2- per cent; Corresponding amounts of benzoyl peroxide furnish ,theioptimum cure for the highly crystalline polyesters.

When the cured polyesterisa rubber-like substance, the incorporation ofa finelydivided reininforcing pigments, appears to inactivate the peroxide,fmaking necessary the use of extremely large amounts of. peroxide tosecure the proper curing. Thus, the incorporation'bf 60 per cent by weight of*Micronex,a channel black, in an unsaturated polyester which would;ordinarily require about 1 per cent benzoyl peroxide for curing may increase the'amount ofperoxide required to about 25 per cent. These large amounts of peroxide produce curedpolyesters which are considerably less stable than those curedwith small amounts, of the order of'several per cent. By the process of the present invention the life cent or 30 per cent or more.

rapidly, andafter about 15 A mixture oi dicarboxylic acids containing 96 mol per-cent sebacic'acidyand, 4 mol per cent,

maleic acid and 25 mol 'per cent excess of a mixture containing 80 mol, per cent isopropylene glycol and 20 mol per cent ethylene glycol, to-

Gether with a small amount ,of zinc chloride as a catalyst, were placed in a closed glass reaction perature above about 125 C; for a timesufliclent to cause anfappreciable improvement in the redistance of the polymer to hydrolysis.

vessel maintained at 200C. and a slow stream of dry, oxygen-free hydrogen was bubbled continuously through the molten mixture'. A packed re-' flux column maintained at 110. C. was attached to the reaction vessel. more water was evolved, indicating that substantially complete esterification had occurred. The reflux column was then removed and the pressure in the system was reduced to. about 6 milli-' meters of mercury, the temperature being main-.

tained at about 200 C. and the bubbling of hydrogen being continued. Glycol distilled over minutes an increase in the viscosity of the product was apparent. At the end of about 8 hours the product was removed and found to be an exceedingly viscous, transparent liquid which crystallized slowly at room temperature to a. tough, flexible, rubbery,

translucent, slightly crystallinesolid, A portion of this solid substance wasquickly reduced to its.

Viscous liquidstate bymillingon cold rolls. About 9 per cent of finely divided benzoyl peroxide was then thoroughly'milled into the gum. Sixtyper cent by weight of Gastex carbon black was then added to the gum on the rolls and the milling was continued until the pigment was well dispersed. This mixture. was then cured under pressure for 10 minutes in'a mold heated to 125 C. in the form of a sheet 50 mils thick. A portion of this "After aboutiive hours no- 2. The method of stabilizing an elastomer produced by curing, with benzoyl peroxide, a polyester prepared by the esterification 01' a reaction mixture consisting of a polymethylenev glycol and isopropylene glycol, the isopropylene glycol constituting at least 80 per cent of the glycol mixture, together with a mixtu'reof dicarboxylic acids;

consisting of sebacicacid and maleic acid, the sebacic acid constituting at least 95 mol per cent of the acid] mixture, the intramolecular.chains formed by said esterification being essentially linear, the number of ester groups insaidpolyester constituting at least 98 per cent of the total number of ester, hydroxyl and carboxyl groups.

I which method comprises heating said cured polymer under a high vacuum at a temperature above 'preciable improvement in the about 125 C. forja time sufiicient to cause an appolymer to hydrolysis.

3. The method "of stabilizing an elastomer produced by ester prepared by the esterification of a reaction mixture consisting of a mixtureof glycols consisting ofequimolecular proportions of ethylene linear, the number of ester groups in sheet was heated at 150 C. for 2 hours under absolute pressure of 6 millimeters of mercury. The treated portion of. the sheet had a life which was about 30 per cent longer than the untreated portion as determined by the time required to reach a given elongation at a given tensile stress upon soaking in water at 80 C.

Although the invention has been described in terms of its specific embodiments, certain modifications and equivalents will .be apparent to those skilled in the art and are intended to be included within the scope of the invention, which is to be limited only, by the reasonable scope of the appended claims.

What is claimed is:

1. The method of stabilizing an elastomer producedby curing, with benzoyl peroxide, a polyester prepared by the esterification of a reaction mixture consisting of a mixture of glycols consisting of about 20 mol per cent of ethylene glycol and about 80 mol per cent of isopropylene glycol and a mixture of dicarboxylic acidsconsisting of about 97 mol per cent of sebacic acid and'about 3 mol per cent of maleic acid, theintramolecular chains formed by said esterificatio'n being essentially linear, the number of-ester groups in said polyester constituting at least 98 per cent of the total number of ester, hydroxyl and carboxyl groups, which method comprises heating said cured polymer under a high vacuum at. a tem- 'glycol'and isopropylene glycol, together. with a mixture of dicarboxylic acids consisting of about 97 mol'per cent succinic and about 3 mol per cent of .maleic acid, the intramolecular chains formed by said esterification being essentially said polyester constituting at least 98 per cent of the total numberfofv ester, hydroxyl and carboxyl groups,

which method comprises heating said cured polymer under ahigh vacuum at a temperature above about'l25C...for a time sufficient to cause an appreciable improvement in the resistance of the polymer to hydrolysis.

4. The method ,of stabilizing an. elastomer produced by curing, with benzoyl peroxide, a polyester prepared by the esterification of a reaction mixture consisting of a mixture of glycols consisting. of isopropylene glycol and polymethylene glycol, the isopropylene glycol constituting at.

least 50 mol per cent of theglycol mixture, to,-

gether with a mixture of .dicarboxylic acids consisting of succinic acid and maleic acid, thesuccinic acid constituting at least mol per cent of the acid mixture, the intramolecular chains formed by said esterification being essentially linear, the number of ester groups in said polyester constituting at least 98 per cent of the total number of ester, hydroxyl and carboxyl groups,

which method comprises heating said cured polymer under a high vacuum at a temperature above about C. for a time sufilcient to cause an appreciable improvement in the resistance of the polymer to hydrolysis.

5. I The method of stabilizing an elastomerproduced by curing, with benzyl peroxide, an isopropylene glycol-ethylene glycol-sebacic acid polyester, the isopropyleneglycol constituent of said polyester constitutingat least 80 mol per cent of the total isopropylene glycol and ethylene glycol, said polyester possessing an intrinsic viscosity in chloroform of at least .4, which method comprises heating said cured polymer under a high vacuum at a temperature above about 125. C. for a time sufficient to cause an appreciable improvement in the resistance of the polymer to hydrolysis.

6. The method of stabilizing an elastomer produced by curing, with benzoyl peroxide, {a polyresistance of the curing, with benzoyl peroxide, a polyester having an intrinsic viscosityot at least .4

ture consisting of a polyinethylene glycol, isopropylene glycol and sebacic acid,- the isopropylene residue in the polyester constituting at said cured polymer under a high vacuum at a temperature above about 125 C. for a time sufficient to cause an appreciable improvement in the resistance of the polymer to hydrolysis.

7. The method of stabilizing an elastomer produced by curing with benzoyl peroxide a. dihy- ;droxyalkane-dicarboxyalkane polyester having 3 an intrinsic viscosity in chloroformofnatleast .4, Y

which method comprises heating said'cured polypolymer to hydrolysis.

' least 80 mol per cent of the total glycol residue in the polyester, which method comprises heatin ester chains of which are linear and are made up of divalent hydrocarbon radicals Joined by ester linkages, the molecules-of said polyester containing on the average less than'five olefinic bonds per 400 atoms in the linear ester chains,

calculated "by assuming no cross-linking between molecules at unsaturated bonds, said polyester containing no, other non-benzenoid unsaturation,

the number of ester groups insaid' polyester constituting'at least 98 per. cent of thetotal number ofester, hydroxyl and carboxyl groups, which method'comprises heating said cured polymer under a high vacuum at; a temperature above about 125 C(for a timesufiicient to cause an apmer under a high vacuum at a temperature above 5 about 125 C. for a time suflicient to cause an ap-- preciable improvement in the resistance of-the 1 8. The method of stabilizing an elastomerpro- 3 3 duced by'curing, with benzoyl peroxide, 9. dihy- 1 idroxyalkane-dicarboxyalkane-maleic acid poll 1 ester, the molecules of said polyester containing 5 i onthe'average less than five 'ol'e'finic bonds per calculated by assuming no cross-linking between the intramolecular ester chains'of saicl polyester heating said cured polymer under a high vacuum the resistance of the polymer to hydrolysis.

duced by curing, with benzoyl peroxide, a glycol- 9. The method of stabilizing an elastomer pro- 400 atomsin' the essentially linear ester chains,

'molecules, at unsaturated bonds, said'polyester containing no other non-benzenoid unsaturation, I

being essentially linear, the number-of ester groups in said polyester constituting at least 98 per cent of the total number oi ester, hydroxyl 1 1 and carboxyl groups, which method comprises 1 at a temperature above about 125 C. for a time 1 sufficient to cause an appreciable improvement in l I xylene glycol-isopropylene; glycol-polymethylene dicarboxylic acid-maleic acid polyester. I

dicarboxylic acid polyester, the intramolecular preciable improvement in the resistance of the polymer to hydrolysis.

10. The method of stabilizingan elastomer pro- 'duced by heating with benzoyl peroxide, to a curing temperature, a polyester prepared. by the esterification of a reactionmixture consisting of about 20' mol per cent of ethylene glycol and about mol per cent of isopropylene glycol and a. mixture of-dicarbox'ylic acids consisting of about 97 mol per cent of sebacic acid and about 3 mol per .cent of maleic acid, the intramolecular chains formed by said esterificati'on being essentially linear, the number'of ester groups in said polyester constituting at least 98 percent of the total number of ester, hydroxyl and carboxyl groups, which method 3 comprises heating. said cured polymer, in a shape possessing a large surface area compared to its volume, at atjemperature above about C. under an absolute pressure of less thanabout 15' millimeters of mercury for at least about two hours. V

11. The method defined in c1aim'8 wherein the glycol-dicarboxylic acid polyester is a polymeth 'IIWBURNARD s. ,BIGGS. 

